1. Field of the Invention
This invention relates to flat cables for the distribution of electrical signals and more particularly to twisted pair flat cables used for the distribution of telecommunications signals in cables which may be positioned under a carpet in an office or other building requiring numerous wiring changes.
2. Description of the Prior Art
Multiconductor flat cables comprising a plurality of separately insulated wires are commonly disposed within an outer insulating jacket to maintain the separate conductors in a flat configuration. One conventional means of forming the outer insulative sheath is to extrude an insulative material, such as polyvinylchloride, around a plurality of conductors. For example, telephone cords interconnected to modular telephone plugs are conventionally manufactured by disposing four side-by-side insulated wires or conductors into an extruded outer plastic sheath. In order to terminate such a flat cable, the outer insulative sheath is merely stripped away adjacent the end of the cable to expose the four insulated conductors. Since the four individual conductors are positioned in a side-by-side relationship, it is quite simple to merely cut the outer insulative sheath transversely around the inner conductors at a point spaced from the end of the cable. The portion of the outer sheath adjacent the end is thus severed from the remainder of the outer sheath and can merely be pulled off of the end of the inner conductors.
Some applications require that the inner conductors be formed as pairs, in which each of the two conductors in each pair is twisted about the other along its length. There are several problems or limitations with the use of a plurality of twisted conductors in a flat cable form. Among these limitations is the fact that twisted pair conductors cannot be easily mass terminated because of the random twist of the conductors, thus requiring the ends of the outer insulative sheath to be removed so that the inner conductors can be deployed. One alternative permitting mass termination of twisted pair conductors is to provide spaced sections in which the conductors are side-by-side and not twisted. One example is shown in U.S. Pat. No. 4,034,148. This approach, however, means that the cable is not uniform along its length and can only be terminated at precise designated locations. Of course, such cable can still be terminated by removing the outer insulative sheath from the cable. However, when an outer insulative sheath is extruded around a plurality of twisted pairs with the outer extruded sheath being in contact with the pairs of conductors along their entire length, it becomes difficult to strip and remove the outer sheath from the twisted pairs. For example, if four twisted-pairs are located in side-by-side relation in an extruded outer sheath, it becomes necessary to slit the sheath along each pair of conductors in order to remove the outer insulation therefrom. Such an operation is both time consuming and labor intensive.
A number of techniques have been suggested which would employ laminated outer insulating layers around a twisted pair. Examples of such laminated twisted pair flat cable are U.S. Pat. Nos. 4,034,148; 4,012,577; and U.S. Pat. No. 4,404,424. Laminated flat cables do, however, present certain disadvantages. For example, lamination of flat cable requires an adhesive or bonding operation and, at least over time, adhesives begin to degrade. Such factors can be especially critical in hostile environments such as under the carpet applications in offices or buildings where a large number of wiring changes can be anticipated.